1. Field of the Invention
The present invention relates to a method of forming a color tube phosphor screen without a phosphor residual, especially a pigment residual.
2. Description of the Related Art
In order to form a phosphor screen of the color tube, after a photoresist is coated, exposed and developed thereby forming a predetermined pattern, a light absorber for increasing the contrast of a phosphor screen is coated thereon. Thereafter, holes are formed at predetermined portions where phosphor layers are subsequently formed, the layers being of three colors.
When the apertures for phosphor layer formation are to be formed, however, it is difficult to completely decompose and remove the photoresist pattern beneath the light absorber. Therefore, the photoresist of a thickness of about 100 .ANG. often remains in the holes. For this reason, when a phosphor slurry of a first color is coated and dried in the holes and then exposed and developed to form a phosphor layer of a first color, phosphor particles of the first color adhere on the residual resist layer in holes for phosphor layers of second and third colors. When the phosphor layers of the second and third colors are formed, therefore, the phosphor particles of the two or more colors are mixed with each other to degrade the color purity.
In order to solve the above problem, Japanese Patent Disclosure (Kokai) No. 56-99945 discloses a method in which after a light-absorbing matrix is formed, a Si.sub.2 O dispersion solution is coated on the entire inner surface of a faceplate and exposed to a HF atmosphere, thereby changing Si.sub.2 O from a sol state to a gel state. This invention provides the treatment against a residual photoresist layer because it is difficult to completely remove the photoresist layer in the holes light-absorbing matrix before phosphor layers are formed. When, for example, PVA is used as a resin component of the photoresist, silica is coated on phosphor particles in order to improve the dispersity of the particles. When PVA and silica are brought into contact with each other, each of PVA and silica on the surfaces of phosphor particles are charged to be (+) and (-), respectively. Therefore, before the phosphor coated with silica is coated on the holes in which the resist layer remains, other silica particles in a gel state are supplied in the holes to adhere therein. Thereafter, the phosphor particles dispersed in the PVA solution are supplied on the faceplate. In this case, the surfaces of the phosphor particles and the surfaces of holes are charged to be (-), since both surfaces are coated with silica particles. Therefore, both surfaces electrically repulse each other. As a result, no phosphor particles remain on the faceplate.
Recently, in order to improve the contrast under ambient light, filters are provided to phosphor layers of the three colors. That is, the phosphor articles are emissive of light in a particular portion of the visible spectrum, and the filter is transmissive of light in those portions of the spectrum and absorptive of light in other portions of the visible spectrum. As a result, the amount of reflected external light from the phosphor layers can be largely reduced without interfering with light emission of each phosphor layer, and an image can be displayed with high contrast. In this case, phosphor particles of each color can be coated with a substance having the above property to form a filter layer.
In using a coating of a slurry of a pigmented phosphor, if a large amount of binder is used so that the pigment is not removed from the phosphor particles, the dispersibility of the phosphor particles is degraded, and pinholes are formed due to coagulation, or contamination occurs due to residual phosphor. For this reason, a binder is not often used, and therefore removal of the pigment cannot be prevented. When the pigment is removed and remains in the holes for another phosphor layer, light emission of another phosphor is interfered with to reduce the luminance and color purity.
In the method disclosed in the aforementioned Japanese Patent Disclosure No. 56-99945, the particle size of the silica particles used in the silica dispersion solution is about 40 nm. When such a silica dispersion solution (in a sol state) is coated on the entire surface of the faceplate and brought into contact with an HF vapor, silica particles which were primary particles in the sol state become two-dimensionally coagulated to form short-chain type huge particles and are scattered to adhere on the faceplate in a gel state, as shown in FIG. 1A. In this method, therefore, a pigment (less than 1.0 .mu.), removed from the phosphor and having a particle size smaller than that of the phosphor particle (several .mu. to 50 .mu.) by one order, enters into gaps between the two-dimensionally coagulated particles and remains in the holes for the phosphor layers.